An envelope tracking system generates an envelope signal that is used as a reference input for a fast switched-mode power supply (Fast SMPS). In turn, the Fast SMPS uses the envelope signal to modulate a supply of a power amplifier for an increased efficiency. At present, an envelope signal generated by traditional methods is not fast or accurate enough for use with the long term evolution (LTE) standard wherein an envelope modulation bandwidth can be as high as 1.5 times a modulation bandwidth. In fact, a 20 MHz LTE bandwidth requires about 30 MHz envelope bandwidth, which further requires a digital sampling clock of 104 MHz for improved oversampling. As a result, there is a need for a method and system that generates fast digital envelope signals using in-phase (I) and quadrature (Q) signals that drive an RF modulator.